1. Field of the Invention
The present invention relates to pharmaceutical dosage forms and in, particular, to a tablet containing a drug having a precisely predetermined release kinetics.
2. Brief Description of the Related Art
There have been various attempts made to create extended release dosage forms for orally administering drugs. Some dosage forms tend to release the drug at rates that do not correlate well with the needs of the patient. For example, a particular dosage form may release a large amount of the drug rapidly upon ingestion where a more constant rate of release is desirable. In other situations, varying release rates may be desirable. Additional background information on the art of controlling release rates of drugs in orally administered dosage forms is found in U.S. Pat. Nos. 5,945,125 and 6,110,500 and U.S. Published Patent Application No. US2005/0025829, the disclosures of which are incorporated herein by reference. Dosage forms typically comprise the drug; that is, the pharmaceutically active substance, dispersed in various excipients including polymers whose rates of dissolution are known. As the tablet is dissolved, the drug is released at a predicted rate. Coating excipients having various rates of dissolution have also been used for time delayed release of drugs.
Common oral extended release or pulsatile release dosage forms include tablets, caplets, and capsules containing small spherical pellets. Such dosage forms typically have the combined geometry of slabs and cylinders, which tend to produce varying release rates. As the tablet is dissolved, the amount of surface area exposed to the dissolution medium changes, thereby changing the rate at which the dissolution occurs and thus the rate of release of the drug. In certain situations, it is desirable that the rate of release of drugs has a zero-order kinetics; that is, the drug is released at a constant or nearly constant rate. However, the rate of drug release from conventional dosage forms typically does not follow zero-order kinetics and thus the drug release rate decreases as release time progresses.
While constant drug release rates are desirable in certain circumstances, it is more generally desirable to be able to customize the kinetics of drug release. For example, a rapid initial release (a burst) may desirably be followed by a period of constant release. In other examples, it might be desirable to delay the release of the drug for a period of time or to release a pulse of the drug after a period of constant release.
As noted above, in order to obtain immediate release followed by extended release or time-delayed release followed by extended release or time-delayed release followed by pulsatile release, dosage forms are coated uniformly with appropriate coating excipients with or without drugs dispersed in the coating. Once the coating layer disappears (or dissolves), the extended release tablet shape is exposed to the dissolution medium and thus the same kinetic problems of other dosage forms are encountered.
These and other problems of the prior art are addressed by the present invention as described following.